1. Technical Field
The present invention relates to a brushless motor for use as a steering-assist motor in an electric power steering apparatus.
2. Description of Related Art
In an electric power steering apparatus which assists steering by driving a steering-assist motor according to a rotating operation of a steering member such as a steering wheel and applying the torque of the motor to a steering mechanism, a brushless motor is widely used as a steering-assist motor (see, for example, Japanese Patent Application Laid-Open No. 6-303752 (1994)).
A brushless motor comprises a cylindrical rotor which is rotatably supported by a rotary shaft in a housing so that it faces the inner side of a stator fixed to the inner circumferential surface of the housing. The rotor comprises a magnet having a plurality of magnetic poles on the outer circumferential surface, while the stator comprises stator coils constructed by winding wires around a plurality of projections, which are provided at equal intervals in a circumferential direction to protrude inward, by using slots between the respective projections. A rotating magnetic field is produced inside the housing by controlling the supply of power to these stator coils, and the torque is applied to the magnet in the rotating magnetic field to rotate the rotor.
The brushless motor is realized by suitably setting a combination of the number of magnetic poles (pole number p) on the outer circumference of the rotor and the number of slots (slot number n) arranged in the inner circumference of the stator. As a steering-assist motor in an electric power steering apparatus, a brushless motor in which the ratio of slot number n to pole number p (=n/p) is within a range of 1.2 to 1.5, such as a brushless motor with 8 poles and 12 slots, or 10 poles and 12 slots, has been widely used.